The BEST Group
The Buffalo Energy Science & Technology Group
MODELING AND SIMULATION OF DYNAMIC VOLTAGE
RESTORER (DVR) FOR VOLTAGE SAG MITIGATION
(OUTLINE)
By
Syed Khundmir T
Department of Electrical Engineering
University at Buffalo
Email: khundmir@buffalo.edu
INTRODUCTION
Electrical power is one of the most dominant factors in our
society.
¢  One major aspect is its quality and stability.
¢  Optimum power quality results in significant increase in
productivity, efficiency and profitability.
¢  Power quality broadly encompasses the study of deviations in
current and voltage waveforms from ideal sine waves.
¢
POWER QUALITY PROBLEMS
A power quality problem can be defined as any deviation of
magnitude, frequency or purity from the ideal sinusoidal
voltage waveform.
CONTD….
1.
2.
There are two major reasons for the increased concern:
Newer-generation load equipment, with microprocessorbased controls and power electronic devices, is more
sensitive to power quality variations than was equipment
used in the past.
Many things are now interconnected in a network. Integrated
processes mean that the failure of any component has much
more important consequences.
For some sensitive devices, a momentary disturbance can
cause scrambled data, interrupted communications, a frozen
mouse, system crashes and equipment failure etc.
CONTD….
Ø
Ø
Ø
Ø
Ø
Ø
Some of the power quality problems are:Voltage sag
Voltage swell
Harmonic distortion
Interruptions
Transients
Surges
Among all the problems, voltage sag and swell are most wide
spread power quality issue affecting distribution systems,
especially industries, where involved losses can reach very
high values. Short and shallow voltage sag can produce
dropout of a whole industry.
VOLTAGE SAG
Definition:
Ø
Ø
Ø
Ø
A decrease of the normal voltage level between 10
and 90% of the nominal rms voltage at the power frequency,
for durations of 0.5 cycle to 1 minute.
In general, voltage sag is the origin of 10 to 90% power
quality problems. The main causes of voltage sag are:Heavy equipment being turned on.
Starting large electrical motors.
Switching power mains.
Overloaded circuits.
VOLTAGE SAG MITIGATION TECHNIQUES
Presently, the most popular techniques to mitigate the effects
of voltage sag are:¢  Dynamic Voltage Restorer (DVR)
¢  Distribution Static Compensator (D-STATCOM)
Among the two techniques mentioned above, DVR is
relatively new technique and provides relatively better voltage
regulation capabilities.
¢  A DVR injects a voltage in series with the system voltage and
a D-STATCOM injects a current into the system to correct the
voltage sag, swell and interruption. It is observed that the
capacity for power compensation and voltage regulation of
DVR is better than D-STATCOM.
DYNAMIC VOLTAGE RESTORER (DVR)
¢
¢
¢
Dynamic Voltage Restorer (DVR) is the most efficient and
effective modern custom power device used in power
distribution networks.
It is normally installed in a distribution system between
the supply and the critical load feeder at the point of
common coupling (PCC).
DVR is designed to protect sensitive equipments like
Programmable Logic Controllers (PLCs), adjustable speed
drives etc from voltage sag and swell.
FEATURES OF DVR
The important features of DVR are:
1.  Lower cost, smaller size, and its fast dynamic response
to the disturbance.
2.
Ability to control active power flow.
3.
Higher energy capacity and lower costs compared to
the SMES device.
4.
Less maintenance required.
LOCATION OF DVR
¢  The
DVR is located at the distribution side of the
power system and helps in the mitigation of
voltage sag.
PRINCIPLE
¢
The basic principle of the DVR is to inject a voltage of
required magnitude and frequency, so that it can restore the
load side voltage to the desired amplitude and waveform even
when the source voltage is unbalanced or distorted.
FUNCTION
A Dynamic Voltage Restorer is basically controlled voltage
source converter that is connected in series with the network.
It injects a voltage on the system to compensate any
disturbance affecting the load voltage.
¢  A basic block diagram for open loop DVR is shown in fig 1:¢
CIRCUIT ANALYSIS
¢
Consider the schematic diagram shown in the above fig 2. By
using KVL
Vth - Zth IL + VDVR = VL
=>VDVR + Vth = VL + Zth IL
MATHEMATICAL EQUATIONS
Therefore, the series injected voltage of the DVR can be
written as
VDVR = VL + Zth IL - Vth
Here,
Vth = system supply voltage (Thevenin voltage)
VL = load bus voltage
Zth = system impedance (Thevenin impedance)
IL = load current
The load power factor angle is given by
¢
CONFIGURATION OF DVR
The main components of the DVR are the units
mentioned below:1.  An Injection/ Booster transformer
2.  A Harmonic filter
3.  Storage Devices
4.  A Voltage Source Converter (VSC)
5.  A Control and Protection system
INJECTION TRANSFORMER
1.
2.
The Injection transformer is a specially designed transformer
that attempts to limit the coupling of noise and transient
energy from the primary side to the secondary side. Its main
tasks are:
It connects the DVR to the distribution network via the HVwindings and transforms and couples the injected
compensating voltages generated by the voltage source
converters to the incoming supply voltage.
In addition, the Injection / Booster transformer serves the
purpose of isolating the load from the system (VSC and
control mechanism).
HARMONIC FILTER
•
•
•
The nonlinear characteristics of semiconductor
devices cause distorted waveforms associated
with high frequency harmonics at the inverter
output.
To overcome this problem and provide high
quality energy supply, a harmonic filtering unit
is used.
This can cause voltage drop and phase shift in
the fundamental component of the inverter
output and has to be accounted for in the
compensation voltage.
STORAGE DEVICES
¢  The
purpose of storage devices is to supply the necessary
energy to the VSC via a dc link for the generation of
injected voltages.
¢  The different kinds of energy storage devices are
Superconductive Magnetic Energy Storage (SMES),
batteries and capacitance.
IMPORTANCE OF ENERGY STORAGE UNITS
(i)
(ii)
During a voltage sag, the DVR injects a voltage to restore the
load supply voltages. It needs a source for this energy.
¢  Two types of system are considered; one using stored energy
to supply the delivered power as shown in Fig. (i), and the
other having no internal energy storage, where energy is taken
from the incoming supply through a shunt converter as shown
in Fig. (ii).
¢
VOLTAGE SOURCE CONVERTER
A VSC is a power electronic system consists of a storage
device and switching devices, which can generate a sinusoidal
voltage at any required frequency, magnitude, and phase
angle.
¢  In the DVR application, the VSC is used to temporarily
replace the supply voltage or to generate the part of the supply
voltage which is missing.
¢  Depending on the method in which DVR takes the input; the
Voltage Source Converter works as either inverter or rectifier
as well as inverter.
¢
CONTROL AND PROTECTION
¢  The
control mechanism of the general
configuration typically consists of hardware with
programmable logic.
¢  All
protective functions of the DVR should be
implemented in the software.
¢  The
performance of DVR is directly affected by
the control strategy of inverter.
OPERATING MODES
The DVR has three modes of operation which are:
1.  Protection mode
2.  Standby mode
3.  Injection/boost mode.
¢
Protection mode:- If the over current on the load side
exceeds a permissible limit due to
short circuit on the load or large
inrush current, the DVR will be
isolated from the systems by using
the bypass switches and supplying
another path for current.
CONTD….
¢  Standby
•
mode:-
In the standby mode the booster transformer’s low voltage
winding is shorted through the converter. No switching of
semiconductors occurs in this mode of operation and the full
load current will pass through the primary.
CONTD….
¢
•
•
Injection/Boost Mode:-In the Injection/Boost mode the DVR
is injecting a compensating voltage through the booster
transformer due to the detection of a disturbance in the supply
voltage.
Due to switching ON of heavy equipment, starting large
electrical motors, switching power mains or overloaded
circuits; voltage sag occurs and reduces the voltage reaching
at the load.
In such case, the DVR is operated in injection mode by
injecting the voltage difference between source voltage and
load voltage at the point of common coupling.
VOLTAGE INJECTION METHODS
Ø
•
Ø
•
There are mainly two different methods of DVR voltage
injection which are
Pre-sag compensation method:
Compensation of voltage sags in the both phase angle and
amplitude sensitive loads would be achieved by pre-sag
compensation method.
In-phase compensation method:
The phase angles of the pre-sag and load voltage are
different but the most important criteria for power quality that
is the constant magnitude of load voltage are satisfied.
PRE-SAG COMPENSATION TECHNIQUE
where,
VDVR = Vprefault – Vsag
ΘDVR = tan-1 [ VL sin θL / (VL cosθL – Vs cosθs )
IN-PHASE COMPENSATION METHOD
¢
One of the advantages of this method is that the amplitude
of DVR injection voltage is minimum for certain voltage sag
in comparison with other strategies.
REFERENCES
¢  M.
Bollen. Understanding Power Quality Problems,
voltage sags and interruptions. IEEE press, 1999.
¢  T.
Jauch, Kara A., Rahmani M., and Westermann D.
Power quality ensured by dynamic voltage correction.
ABB Review, Vol. 4:pp. 25 – 36, 1998.
¢  BENACHAIBA Chellali,
FERDI Brahim, Voltage
Quality Improvement Using DVR, Electrical Power
Quality and Utilization, Journal Vol. XIV, No. 1, 2008